1. Field of the Invention
The present invention relates to an amplifying device, and more particularly to an improvement for achieving both larger dynamic range and easier control of idling current.
2. Description of the Background Art
In many cases, a power amplifying device has a configuration of a push-pull type class AB amplifying device in which two transistors are combined. A class AB amplifying device, comprising a transistor for drawing an output current out from a positive power supply line to a load and a transistor for drawing the output current from the load into a negative power supply line, alternately turns the two transistors on to achieve draw-out and draw-in of the output current and turns both the transistors on in some degree at switching between the draw-out and draw-in.
Therefore, the class AB amplifying device has an advantage of saving a consumption current since only a very small amount of idling current flows from one transistor to the other when the output current is zero. Further, since the two transistors do not turn off simultaneously, it is advantageously possible to suppress crossover distortion and improve switching characteristics.
A background-art class AB amplifying device using MOSFETs has a configuration of combining a source follower of an n-channel type MOSFET and that of a p-channel type MOSFET. FIG. 26 is a circuit diagram showing an exemplary configuration of such a background-art class AB amplifying device. In the device 150, an n-channel type MOSFET 161 and a p-channel type MOSFET 162 of which source electrodes are connected to each other are disposed between a positive power supply line 163 and a negative power supply line 164. An output terminal 165 is connected to a node between the two source electrodes.
Further, between a positive power supply line 170 and a negative power supply line 171 disposed is a series circuit in which a resistance element 169, an n-channel type MOSFET 166, a p-channel type MOSFET 167 and an n-channel type MOSFET 168 are connected in this order. Drain electrodes of the MOSFETs 166 and 167 are connected to each other.
A gate electrode and a source electrode of the MOSFET 166 are connected to each other and similarly a gate electrode and a source electrode of the MOSFET 167 are connected to each other. A node between the resistance element 169 and the source electrode of the MOSFET 166 is connected to a gate electrode of the MOSFET 161, and a node between a drain electrode of the MOSFET 168 and the source electrode of the MOSFET 167 is connected to a gate electrode of the MOSFET 162. Further, an input terminal is connected to a gate electrode of the MOSFET 168.
When a voltage to make the MOSFET 168 full-on is applied as an input voltage Vin, a large amount of current flows in the resistance element 169 through the MOSFETs 166 and 167. That increases a voltage drop across the resistance element 169, and hence gate voltages of the MOSFETs 161 and 162 drop. As a result, the MOSFET 161 turns off and the MOSFET 162 turns on. An output current is thereby drawn from a load into the negate power supply line 164 through the output terminal 165.
On the other hand, when a voltage to make the MOSFET 168 full-off is applied as the input voltage Vin, only a small amount of current flows in the resistance element 169. That decreases the voltage drop across the resistance element 169, and hence the gate voltages of the MOSFETs 161 and 162 rise. As a result, the MOSFET 161 turns on and the MOSFET 162 turns off. The output current is thereby drawn out from the positive power supply line 163 to the load through the output terminal 165.
Thus, in response to the input voltage Vin, draw-out of the output current (current source) and draw-in thereof (current sink) are performed. The MOSFETs 166 and 167 serve to create a potential difference between the gate electrodes of the MOSFETs 161 and 162. At switching between the current sink and the current source, a current proportional to a current flowing in the MOSFETs 166 and 167 flows from the MOSFET 161 to the MOSFET 162 as an idling current. That achieves a class AB operation where the MOSFETs 161 and 162 do not turn off simultaneously.
The device 150 has a problem in being incorporated as an IC (Integrated Circuit) in a single semiconductor chip as follows. In the IC, usually, the negative power supply lines 164 and 171 are formed as a common ground power supply line. That causes a problem that an output voltage Vout can not be less than a value obtained by adding a source-drain voltage of the MOSFET 168 and the gate-source voltage of the MOSFET 162 when the MOSFET 168 is made full-on to a negative power supply potential -Vcc of the negative power supply line 164.
Specifically, a dynamic range of the output voltage Vout is disadvantageously limited smaller than a potential difference (power supply voltage) between a positive power supply potential Vcc and the negative power supply potential -Vcc. In a largely-rated amplifying device, a gate-source voltage to make the MOSFET full-on is high and when the power supply voltage is low, a ratio of lost dynamic range to the power supply voltage is not negligible. The problem is pronounced when an IC is used for portable electronics using battery as power supply.
As a solution of this problem known is an amplifying device comprising two n-channel type MOSFETs connected in series between a positive power supply line and a negative power supply line, two preliminary amplifiers to separately control these two MOSFETs and another MOSFET to prevent these two MOSFETs from turning on simultaneously, to achieve a class AB operation. This device, however, has a problem that it is not easy to control the idling current of the two MOSFETs and switching distortion and a through current (excessive idling current) are likely to be caused.